Movie piracy is a cause of substantial revenue loss to the motion picture industry. Illegally copied movies, filmed during projection with video cameras or camcorders and similar devices, are a significant contributing factor to revenue loss. Even the questionable quality of copies pirated in this fashion does not prevent them from broad distribution in the “black market”, especially in some overseas markets, and on the Internet. As video cameras improve in imaging quality and become smaller and more capable, the threat of illegal copying activity becomes more menacing to motion picture providers. While it may not be possible to completely eliminate theft by copying, it can be advantageous to provide display delivery techniques that frustrate anyone who attempts to copy a motion picture using a portable video camera device.
It is known to provide a distinct symbol or watermark to an original still image as a means of image or copy identification, such as in order to authenticate a copy. As examples, U.S. Pat. No. 5,875,249 (Mintzer et al.), U.S. Pat. No. 6,031,914 (Tewfik et al.), U.S. Pat. No. 5,912,972 (Barton), and U.S. Pat. No. 5,949,885 (Leighton) disclose methods of applying a perceptually invisible watermark to image data as verification of authorship or ownership or as evidence that an image has not been altered. However, where such methods identify and validate image data, they provide no direct means of protection against copying an image, such as using a conventional scanner and color printer. In contrast, U.S. Pat. No. 5,530,759 (Braudaway et al.) discloses providing a visible, color correct watermark that is generated by altering brightness characteristics but not chromaticity of specific pixels in the image. But the approach used in U.S. Pat. No. 5,530,759 could be objectionable if used for a motion picture, since the continuing display of a watermark on film could annoy an audience and adversely affect the viewing experience.
The above examples for still-frame images illustrate a key problem: an invisible watermark identifies but does not adversely affect the quality of an illegal copy, while a visible watermark can be distracting and annoying. With video and motion picture images, there can be yet other problems with conventional image watermarking. For example, U.S. Pat. No. 5,960,081 (Vynne et al.) discloses applying a hidden watermark to MPEG data using motion vector data. But this method identifies and authenticates the original compressed data stream and would not provide identification for a motion picture that was copied using a camcorder. Other patents, such as U.S. Pat. No. 5,809,139 (Girod et al.), U.S. Pat. No. 6,069,914 (Cox), and U.S. Pat. No. 6,037,984 (Isnardi et al.) disclose adding an imperceptible watermark directly to the discrete cosine transform (DCT) coefficients of a MPEG-compressed video signal. If such watermarked images are subsequently recompressed using a lossy compression method (such as by a camcorder, for example) or are modified by some other image processing operation, the watermark may no longer be detectable.
The invisible watermarking schemes disclosed in the patents listed above add a watermark directly to the compressed bit stream of an image or image sequence. Alternatively, there are other watermarking schemes that add the watermark to the image data itself, rather than to the compressed data representation. An example of such a scheme is given in U.S. Pat. No. 6,044,156 (Honsinger et al.), which discloses a spread spectrum technique using a random phase carrier. However, regardless of the specific method that is used to embed a watermark, there is always a concern that a watermarking method be robust, that is, able to withstand various “attacks” that can remove or alter the watermark. Some attacks may be deliberately aimed at the underlying structure of a given watermarking scheme and require detailed knowledge of watermarking techniques applied. However, most attack methods are less sophisticated, performing common modifications to the image such as using lossy compression, introducing lowpass filtering, or cropping the image, for example. Such modifications can be made when a video camera is used to capture a displayed motion picture. These methods present a constant threat that a watermark may be removed during the recording process.
The watermarking schemes noted above are directed to copy identification, ownership, or authentication. However, even if a watermarking approach is robust, provides copy control management, and succeeds in identifying the source of a motion picture, an invisible watermark may not be a sufficient deterrent for illegal copying.
As an alternative to watermarking, some copy deterrent schemes used in arts other than video or movie display operate by modifying a signal or inserting a different signal to degrade the quality of any illegal copies. The modified or inserted signal does not affect playback of a legally obtained manufactured copy, but adversely impacts the quality of an illegally produced copy. As one example, U.S. Pat. No. 5,883,959 (Kori) discloses deliberate modification of a burst signal to foil copying of a video. Similarly, U.S. Pat. No. 6,041,158 (Sato) and U.S. Pat. No. 5,663,927 (Ryan) disclose modification of expected video signals in order to degrade the quality of an illegal copy. As yet another example of this principle, U.S. Pat. No. 4,644,422 (Bedini) discloses adding a degrading signal to discourage copying of audio recordings. An audio signal having a frequency at and above the high threshold frequency range for human hearing is selectively inserted into a recording. The inserted signal is not detectable to the listener. However, any unauthorized attempt to copy the recording onto tape obtains a degraded copy, since the inserted audio signal interacts adversely with the bias oscillator frequency of a tape recording head.
The above-mentioned copy protection schemes disclose the use of a deliberately injected signal introduced in order to degrade the quality of an electronic copy. While such methods may be effective for copy protection of data from a tape or optical storage medium, these methods do not discourage copying of a motion picture image using a video camera.
As a variation of the general method where a signal is inserted that does not impact viewability but degrades copy quality, U.S. Pat. No. 6,018,374 (Wrobleski) discloses the use of a second projector in video and motion picture presentation. This second projector is used to project an infrared (IR) message onto the display screen, where the infrared message can contain, for example, a date/time stamp, theater identifying text, or other information. The infrared message is not visible to the human eye. However, because a video camera has broader spectral sensitivity that includes the IR range, the message will be clearly visible in any video camera copy made from the display screen. The same technique can be used to distort a recorded image with an “overlaid” infrared image. While the method disclosed in U.S. Pat. No. 6,018,374 can be effective for frustrating casual camcorder recording, the method has some drawbacks. A more sophisticated video camera operator could minimize the effect of a projected infrared watermark using a filter designed to block infrared light. Video cameras are normally provided with some amount of IR filtering to compensate for silicon sensitivity to IR. With a focused watermark image, such as a text message projected using infrared light, retouching techniques could be applied to alter or remove a watermark, especially if the infrared signal can be located within frame coordinates and is consistent, frame to frame. A further drawback of the method disclosed in U.S. Pat. No. 6,018,374 relates to the infrared light source itself. Since an infrared lamp can generate significant amounts of heat, it may not be practical to project a watermark or copy deterrent image over a large area of the display screen using only an IR source.
Motion picture display and video recording standards have well-known frame-to-frame refresh rates. In standard motion picture projection, for example, each film frame is typically displayed for a time duration of 1/24 second. Respective refresh rates for interlaced NTSC and PAL video recording standards are 1/60 second and 1/50 second. Video camera capabilities such as variable shutter speeds allow close synchronization of a video camera with film projection, making it easier for illegal copies to be filmed within a theater. Attempts to degrade the quality of such a copy include that disclosed in U.S. Pat. No. 5,680,454 (Mead). U.S. Pat. No. 5,680,454, which discloses use of a pseudo-random variation in frame rate, causing successive motion picture frames to be displayed at slightly different rates than nominal. Using this method, for example, frame display periods would randomly change between 1/23 and 1/25 second for a nominal 1/24 second display period. Timing shifts within this range would be imperceptible to the human viewer, but significantly degrade the quality of any copy filmed using a video camera Randomization, as used in the method of U.S. Pat. No. 5,680,454, would prevent resynchronization of the video camera to a changed display frequency. While the method of U.S. Pat. No. 5,680,454 may degrade the image quality of a copy made by video camera, this method does have limitations. As noted in the disclosure of U.S. Pat. No. 5,680,454, the range of frame rate variability is constrained, since the overall frame rate must track reasonably closely with accompanying audio. Also, such a method does not provide a mechanism for including any type of spatial pattern or watermark in each frame, which could be used to provide a human-readable warning message or to trace the individual copy of the film that was illegally recorded.
U.S. Pat. No. 5,959,717 (Chaum) also discloses a method and apparatus for copy prevention of a displayed motion picture work. The apparatus of U.S. Pat. No. 5,959,717 includes a film projector along with a separate video projector. The video projector can be used, for example, to display an identifying or cautionary message or an obscuring pattern that is imperceptible to human viewers but can be recorded using a video camera Alternately, the video camera may even display part of the motion picture content itself. By controlling the timing of the video projector relative to film projector timing, a message or pattern can be made that will be recorded when using a video camera, but will be imperceptible to a viewing audience. The method of U.S. Pat. No. 5,959,717, however, has some drawbacks. Notably, this method requires distribution of a motion picture in multiple parts, which greatly complicates film replication and distribution. Separate projectors are required for the film-based and video-based image components, adding cost and complexity to the system and to its operation. Image quality, particularly for large-screen environments, may not be optimal for video projection and alignment of both projectors to each other and to the display surface must be precisely maintained.
Conventional methods such as those described above could be adapted to provide some measure of copy deterrence and watermarking for digital motion pictures. However, none of the methods noted above is wholly satisfactory, for the reasons stated. None of the existing copy protection or watermarking methods takes advantage of key characteristics of the digital motion picture environment that would prevent successful recording using a video camera.
While the capability for encoding “passive” invisible digital watermarks within digital images data has been developed, there is a need for more aggressive copy-deterrence techniques that can be embedded within digital motion picture data content and can take full advantage of digital projector technology.
Image aliasing is a well-known effect that results from a difference between the scan line or frame refresh rate of an electronic display or motion picture and the sampling rate of a video camera. Inherently, image aliasing imposes some constraints on the image quality of a video camera recording made from a display screen. Thus it is known that simply varying a scan or refresh rate may result in increased levels of aliasing. For example, video projectors from Silicon Light Machines, Sunnyvale, Calif., use a high scan rate and complex segmented scanning sequence that can corrupt a video-taped copy by producing vertical black bars in the captured image. Similar effects are also observed when one tries to capture an image from a computer screen with a camcorder. These are the result of differences in scan rates between the display and the video camera systems. These techniques, however, offer a somewhat limited capability for protection, since scan synchronization of video camera apparatus makes it feasible to override this protection. Moreover, aliasing caused by simple scan rate differences does not provide a suitable vehicle for display of a warning message or other pattern in a taped copy or for digital watermarking in order to identify the source of the original image.
In a fully digital motion picture system, the spectral content and timing of each displayed pixel is known and can be controlled for each frame. While there can be a standard refresh rate for screen pixels (corresponding to the 1/24 or 1/30 second frame rate used for motion picture film or video displays), there may be advantages in altering the conventional “frame-based” model for motion picture display. Each displayed pixel on the screen can be individually addressed within any frame, and its timing characteristics can be modified as needed. This capability has, however, not been used for displaying a copy-deterrent pattern.
Therefore, it can be seen that there is clearly a need for a method that allows embedding of a copy-deterrent pattern within motion picture content, where the content is projected from digital data. It would be most advantageous for such a pattern to be invisible to a viewer but recordable using a video camera. Further, it can be seen that there is a need for a method that uses the opportunity for control of timing and of individual screen pixel content that digital motion picture technology offers in order to discourage movie piracy using a video camera.